Add System.find_executable!/1

[wojtekmach]

No description provided.

[josevalim]

This is one that I don't mind not having a bang variant because:

1. This one can legitmitely always fail. It is very hard to say with 100% confidence we will find an executable, so there could always be something outside of your control
2. You can easily add a || raise ... that has more semantic meaning to your project/app

Thanks!